Currently, there are numerous support and rotation mechanisms for solar tracker structures, which can be classified depending on the angular travel they offer, on their load capacity both in terms of retention and actuation, and on their precision in tracking the sun. And the clear objective of all of them is to reduce their manufacturing, assembly and maintenance costs for required features.
The azimuthal rotation mechanism is especially difficult and costly, regarding which U.S. Pat. No. 6,123,067 and WO2013/178850 A1, based on the actuation of two hydraulic cylinders, can be cited as background art.
The mechanism subject of U.S. Pat. No. 6,123,067 comprises a rotating frame that rotates around the pedestal, actuated by two hydraulic cylinders. This mechanism is complex, costly and exhibits the problem of large gaps, which require constant maintenance.
Patent application WO 2013/178850 A1 discloses an azimuthal hydraulic actuation mechanism materialized by means of two hydraulic cylinders attached to the same common shaft, but at a different height so that they do not cross, allowing a complete 360° rotation of the solar panel structure with respect to the support pedestal. The problem with this azimuthal mechanism is that its load capacity is too variable and imbalanced during its circumferential travel, such that in order to absorb the load of wind that the supporting structure can receive in any direction, it requires a significant oversizing of the entire mechanism, both for its actuating load capacity required to move the structure and for withstanding the wind without moving.
In addition to the load imbalance generated along the length of the azimuthal travel of the tracker by the actuation through two hydraulic cylinders, given their relative position with respect to each other and which is usually optimized in an angular separation between them of approximately 90°, there is another imbalance generated on the common shaft the cylinders are attached to, by having their ends arranged one on top of the other and causing an additional bending moment on the shaft with it. This load imbalance, derived from the relative position of the two cylinders with respect to each other on the one hand, and on the other, the fact that the ends of the cylinders are attached to the same common shaft but one on top of the other, requires having spherical ball-and-socket joints at the ends of these cylinders in their attachment to the shaft. These spherical ball-and-socket joints, which are expensive and occupy a lot of space, are arranged to absorb deformation deviations in the system by introducing imbalanced and off-centered loads to prevent the cylinders from breaking or a significantly reduced durability of the same and of their attachments to the rest of the azimuthal mechanism. Likewise, the mechanical rotating element between the movable part of the solar tracker and the fixed pedestal, which is usually a ball bearing, also experiences much greater loads as a consequence of the load imbalance transmitted by the cylinders to the common shaft they are attached to, requiring the oversizing of this mechanical rotating element, which is typically a ball bearing or a journal bearing, to attempt to minimize its greater wear while in operation and to achieve an acceptable degree of durability.
Document WO 2008/148919 A2 discloses system based on a hydraulically operated kinematic rod-crank mechanism including a tower which terminates downward in a moving ring which rotates in relation to a stationary ring by means of four hydraulic pistons in order to produce an azimuth movement. The tower terminates upward in a shaft parallel to the ring, on which shaft a sail, bearing solar collectors and actuated by two hydraulic pistons, can pivot in order to produce a zenith movement.
In central tower and heliostat field solar thermal plants, economies of scale to reduce the costs of generating electricity are resulting in increasingly larger plants, which require thousands of large heliostats leading to a solar field configuration of 360° surrounding the tower. For this type of solar fields, the angular travel in the azimuth shaft required for the heliostats is quite high, especially for heliostats situated in the southern area for solar plants located in the northern hemisphere of the earth, to the extent they require a complete rotation in order to track the sun at all times and avoid downtimes during the operation of the plant. In a wide angular travel azimuthal mechanism, provided with a limit switch, the effective azimuth rotation can be approximately 350°, that is to say, almost but not a complete rotation. In these cases and for the southern heliostat field, there are two types of singularities that take place during the operation of the plant when the heliostats track the sun. One of them is the singularity that could be called the azimuth blind spot singularity due to the failure to rotate completely, such that when the heliostat reaches its limit, it must turn around to be able to position itself to track the sun once again, losing its availability while engaging in said maneuver, which can last for about 30 minutes. However, there is another drawback which has a greater impact on the downtime of the heliostats in the southern field, which consists in the failure by the heliostats in the southern field to position themselves with a very low elevation angle, that is to say, too horizontally, to track the sun and send its reflected energy to the receiver located at the highest point of the tower at the tracking speed, which has to be sufficiently low to track the sun with the correct precision. In this case, the heliostat has to reposition itself by activating an emergency speed, which is much faster, to seek its closest tracking position to minimize its downtime.
This downtime may be minimized by providing the azimuthal mechanism with the ability to rotate completely, beyond 360°, as the elevation rotation range usually is limited to 90° or a bit more, to avoid making the actuation on the elevator shaft unnecessarily complicated and costly.
The hydraulic actuation in the azimuth shaft, based on hydraulic cylinders, is very attractive due to its reliability and low cost, and due to its high load capacity; however, its physical materialization is the key to achieve a high rotation capacity, beyond 360° and to work with a balanced load capacity along the length of its rotation to optimize the mechanism, and with it, the size of its hydraulic cylinders.